One conventional image forming device provided with a fixing unit for thermally fixing a toner image formed on a sheet has the following structure (see Japanese Patent Application No. 2011-242746). This conventional image forming device has a temperature detection unit that detects the temperature of the fixing unit. At the beginning of a printing operation, the image forming device determines whether the number of sheets to be printed is greater than a prescribed number and enters either (1) a first mode for supplying the first sheet to be printed once the temperature detected by the temperature detection unit exceeds a first temperature when the number of sheets is greater than the prescribed number or (2) a second mode for supplying the first sheet to be printed once the detected temperature exceeds a second temperature lower than the first temperature when the number of sheets is no greater than the prescribed number.